At the present time, solvent-dominated recovery processes (SDRPs) are rarely used to produce highly viscous oil. Highly viscous oils are produced primarily using thermal methods in which heat, typically in the form of steam, is added to the reservoir. Cyclic solvent-dominated recovery processes (CSDRPs) are a subset of SDRPs. A CSDRP is typically, but not necessarily, a non-thermal recovery method that uses a solvent to mobilize viscous oil by cycles of injection and production. Solvent-dominated means that the injectant comprises greater than 50% by mass of solvent or that greater than 50% of the produced oil's viscosity reduction is obtained by chemical solvation rather than by thermal means. One possible laboratory method for roughly comparing the relative contribution of heat and dilution to the viscosity reduction obtained in a proposed oil recovery process is to compare the viscosity obtained by diluting an oil sample with a solvent to the viscosity reduction obtained by heating the sample.
In a CSDRP, a viscosity-reducing solvent is injected through a well into a subterranean viscous-oil reservoir, causing the pressure to increase. Next, the pressure is lowered and reduced-viscosity oil is produced to the surface through the same well through which the solvent was injected. Multiple cycles of injection and production are used. In some instances, a well may not undergo cycles of injection and production, but only cycles of injection or only cycles of production.
CSDRPs may be particularly attractive for thinner or lower-oil-saturation reservoirs. In such reservoirs, thermal methods utilizing heat to reduce viscous oil viscosity may be inefficient due to excessive heat loss to the overburden and/or underburden and/or reservoir with low oil content.
References describing specific CSDRPs include: Canadian Patent No. 2,349,234 (Lim et al.); G. B. Lim et al., “Three-dimensional Scaled Physical Modeling of Solvent Vapour Extraction of Cold Lake Bitumen”, The Journal of Canadian Petroleum Technology, 35(4), pp. 32-40, April 1996; G. B. Lim et al., “Cyclic Stimulation of Cold Lake Oil Sand with Supercritical Ethane”, SPE Paper 30298, 1995; U.S. Pat. No. 3,954,141 (Allen et al.); and M. Feali et al., “Feasibility Study of the Cyclic VAPEX Process for Low Permeable Carbonate Systems”, International Petroleum Technology Conference Paper 12833, 2008.
The family of processes within the Lim et al. references describes embodiments of a particular SDRP that is also a cyclic solvent-dominated recovery process (CSDRP). These processes relate to the recovery of heavy oil and bitumen from subterranean reservoirs using cyclic injection of a solvent in the liquid state which vaporizes upon production. The family of processes within the Lim et al. references may be referred to as CSP™ processes.
One complication of using light solvents is that they readily form gas clathrates at high pressures, such as those existing in subsurface oil reservoirs, and at lower temperatures, such as can exist in shallow reservoirs in cool climates (e.g., bitumen reservoirs in Alberta, Canada). Gas clathrates, which are also referred to as “gas hydrates” or just “hydrates”, are similar to water ice and comprise solid-phase water in which one of several lattice structures act as the molecular cages to trap to ‘guest’ molecules. Hydrates can be formed with many ‘guest’ molecules, however, it is the hydrates of methane, ethane, propane, butane, and carbon dioxide which are of greatest importance for this discussion. The conditions at which hydrates will form depend on many factors including temperature, pressure, and composition. Hydrates are well known to be stable over a wide range of high pressures (generally at least several atmospheres) and near ambient temperatures (as described, for instance, in Katz et al.; Handbook of Natural Gas Engineering; McGraw-Hill Bk. Co., p. 212; 1959). Specific hydrate formation conditions are composition dependent. For example, methane forms solid hydrates with pure water at temperatures above 0° C. at pressures greater than about 2.5 MPa, whereas propane forms solid hydrates with pure water at temperatures of about 0° C. at pressures greater than about 0.16 MPa.
Hydrates may be either naturally occurring or man-made. Man-made hydrates are typically created during oil and gas production and processing when the phase boundary of hydrates is unintentionally encroached. Man-made hydrates are often a nuisance due to their tendency to plug pipes and equipment. If hydrates are inadvertently formed in situ during recovery of oil and gas, significant reduction of productivity may occur. This may be a particular issue if low molecular weight solvents, e.g., ethane, propane, or carbon dioxide, are injected into relatively cold oil-bearing formations to aid productivity.
Lim et al. in U.S. Pat. No. 6,769,486 and Canadian Patent No. 2,349,234 disclose a cyclic solvent process for in situ bitumen and heavy oil production. In the process, a light hydrocarbon solvent, such as ethane or propane, is injected in a liquid-phase into the reservoir and produced through a common wellbore at least in part in a vapor-phase. Lim et al. teaches using a hydrate inhibitor to prevent hydrates in wellbores and “that conditions of the oil sand reservoirs are such that hydrates are less likely to form in the reservoir during injection and production phases.” However, under some reservoir conditions, hydrate formation can reduce permeability, especially in the near-wellbore region. Lim et al. disclose that the solvent may be injected in a heated state in a preferred temperature range of 10-50° C. However, Lim et al. does not teach that this is done or optimized for controlling hydrates, especially in situ. Moreover, methods to assess how much heat to add are not disclosed. Thus, a need exists to limit or prevent hydrate formation within an oil reservoir undergoing cyclic solvent injection, especially in the near-wellbore region. Moreover, there is a need to do this in a manner to minimize cost and energy usage.